This invention relates to a method of and means for scanning a body with penetrating radiation to enable a cross-section thereof to be reconstructed in terms of the distribution over the cross-section of the coefficients of absorption with respect to such radiation.
U.S. Pat. No. 3,934,142 shows a scanner for obtaining such a distribution utilizing a scanner head assembly comprising a point source of penetrating radiation and a planar array of detectors spatially fixed with respect to the source and located at discrete angular positions with respect to the source. Those beams from the source incident on the detectors establish a plurality of discrete, coplanar pencil beams of penetrating radiation defining a planar fan beam.
The head assembly is positioned so that the medial beam of the fan beam passes through a fixed center about which the head assembly is rotatable in the plane of the detectors. During a complete revolution of the head assembly, the source moves in a circle concentric with the fixed center and the fan beam sweeps out an area, in the plane of the detectors, over which the absorption coefficients with respect to the penetrating radiation can be computed by any of several known reconstruction algorithms, such as those described in reference [2], operating on the outputs of the detectors. The area swept out by the fan beam is contained within a circle, termed the circle of reconstruction, such circle being coplanar with the fan-beam and having its center coincident with the fixed center and its periphery tangent to the beams defining the extremities of the fan beam. In a given scanner, the diameter of the circle of reconstruction is directly related to the apical angle of the fan beam, i.e., the angle between the extremities of the fan beam.
When a living body is placed in the circle of reconstruction, a cross-section through the body in the plane of the circle of reconstruction can be reconstructed using the scanning technique described above. In order to minimize the radiation dosage applied to the living body, all beams from the source except those incident on the detectors are blocked by a pre-collimator interposed between the source and the body. When a section through a non-living body, such as an industrial object, is to be reconstructed, the pre-collimator is optional. Regardless of the nature of the body being examined, only those beams incident on the detectors contribute to the outputs of the detectors. For this reason, the term "fan-beam of the type described", as used hereinafter, means either a group of discrete coplanar pencil beams of penetrating radiation produced by a collimator to which a fan of radiation from a point source is applied, or those discrete coplanar beams of a fan of radiation produced by a point source and incident on a planar array of detectors when no collimator is used. Furthermore, the term "penetrating radiation" as used hereinafter, is a term of convenience intended to cover radiation that penetrates such as, but not limited to, X-rays from an X-ray tube as well as gamma rays from a nuclear source. Finally, unless otherwise specified, the term "intensity of a beam of a fan beam" means the intensity of the beam incident on a detector and is thus used to designate the intensity of a beam emerging from the area swept out by the fan beam during its rotation relative to a fixed center, as distinguished from the intensity of the beam produced by the source. A beam emerging from such area may be attenuated or not depending upon whether the beam has passed through a body located in the area.
A scanner having a head assembly that produces a fan beam of the type described is, hereinafter, termed a scanner of the type described. When a scanner of the type described is arranged so that the head assembly is rotatable about a spatially fixed center, the scanner is said to have a scan utilizing pure rotational movement. Such a scan carried out by a scanner of the type described, wherein the medial beam of the fan beam passes throught the fixed center is, hereinafter, termed a scan of the type described.
The physical size of a scanner of the type described, which carries out a scan of the type described, is dependent on the size of the circle of reconstruction which, as pointed out above, is dependent on the apical angle of the fan beam. The size of the circle of reconstruction is dictated by the largest body to be scanned; and the apical angle of the fan beam is dictated by the nature of the source to be used. In medical equipment for scanning a human torso, the circle of reconstruction must be about 500 mm in diameter, while for scanning a human head, a circle of reconstruction of about 250 mm is adequate. The apical angle of conventional X-ray tubes, presently used as the source in conventional scanners of the type described, is about 40.degree. requiring the source to be rotatable along a circular path about two meters in diameter for a torso scanner. When the supporting structure for the head assembly is taken into account, the scanner stands about three meters high by about three meters wide, a rather formidable piece of equipment to say the least. Furthermore, some scanners use as many as 300 detectors, which, with their associated electronic circuitry, establish a significant mass that must be supported and rotated.
Compared to a torso scanner, a human head scanner with a 40.degree. fan beam, would be about half the physical size. While a finer fan beam is usually employed in a head scanner to improve picture quality and provide more details in the reconstructed cross-section as compared to a torso scanner, only about two-thirds as many detectors are required as compared to a torso scanner. Thus, a head scanner is smaller, lighter and less expensive than a torso scanner.
Torso scanners and head scanners of the type described have been built and are in use for their specialized purposes. A head scan could be carried out in a torso scanner, since its circle of reconstruction is large enough to accommodate a human head; but this is usually not done because the picture quality obtained would be too poor. On the other hand, if a torso scan were required with picture quality comparable to that obtained from a head scanner, it is apparent that the availability of a head scanner would be irrelevant because its circle of reconstruction could not accommodate a torso.
A large medical facility with a need for the ultimate in diagnostic equipment will thus require two separate scanners of the type described, each limited to different portions of a human body. Each scanner is obviously physically large, tremendously expensive, and unfortunately inflexible in use, which is a factor that adds to the financial burden on a medical facility. And notwithstanding all the expense, the picture quality obtained with a torso scanner is not comparable to that obtained with a head scanner.
It is an object of the present invention to provide a new and improved scanner of the type described, and a new and improved scanning technique utilizing pure rotational movement of the head assembly wherein the above-described deficiencies of known scanners of the type described are overcome or substantially reduced.